This application relates to a freeze dryer for histological use, and in particular, to a tissue freeze dryer for the preparation of paraffin or resin embedded tissues in a form suitable for easy storage and handling.
The preparation of tissue specimens in a form suitable for storage and study has traditionally been performed in a multitude of separate and discrete steps, each step requiring extensive handling with the attendant increased chances of contamination and other problems. In general, preparation of a tissue specimen involves the rapid removal of the tissue from the animal, freezing the tissue specimen at temperatures from -60.degree. C. to -130.degree. C. The frozen specimen is then dried by placement in a vacuum where the vacuum sublimates the water from the sample; liquid degassed paraffin or resin is then allowed to completely engulf and permeate the tissue specimen. The resin or paraffin cools and solidifies, after which the embedded tissue is removed and sectioned by a microtome or other means for study.
Should formaldahyde-induced-fluorescence (FIF) be desired, paraformaldahyde gas is allowed to permeate the vacuum chamber to allow absorption of the gas by the resin or paraffin.
To eliminate the problems of contamination or the like associated with performing these various procedures in separate operations, the apparatus of the type disclosed in U.S. Pat. No. 3,639,999 and that disclosed in Ser. No. 746,294, filed Dec. 1, 1976, now abandoned were developed. These assemblies allow the preparation of a resin embedded specimen to be accomplished within one assembly without the necessity of transference to another container or any other unnecessary handling once the tissue specimen has been cut from the animal. In assemblies of the type disclosed in the above art, the tissue sample is placed in a cup within the assembly; the degassed resin or paraffin is floated about the sample and solidified. The embedded specimen may then be removed from the assembly and sliced with a microtome for study, or it may be stored for future study in this condition.
In these tissue embedding assemblies of the prior art, a tissue support plate receives and supports a cup holder which has openings integrally formed for the reception of tissue holding cups. The holder is removable from the support plate to allow the easy removal and/or placement of the tissue holding cups in the opening. The support plate itself is heated through an internal resistance heater, connected to a source of power by wires passing through the cover. When embedding with paraffin, chunks of paraffin are dispersed on the support plate and, when melted, seep into perforated tissue holding cups to embed the tissue specimens. Hot spots and poor temperature control in the support plate are unavoidable when such a support plate is heated internally, as taught in the above-identified patents. These hot spots can cause foaming and decomposition of the paraffin, which in turn leads to deposits of paraffin decomposition products which can form on the support plate. These deposits decrease the heating efficiency, create further hot spots and can contaminate the embedded tissue specimens. With such apparatus of the prior art, the support plate was not easily removed for cleaning while its location within the freeze drying chamber did not permit effective cleaning within the chamber.